In a magnetic field measurement device utilizing the magneto-optical effect due to optical pumping, a glass cell in which an alkali metal gas (such as potassium, rubidium, or cesium, for example) is enclosed is used as a sensor. A static magnetic field is applied to the glass cell, the energy level of alkali metal in the glass cell is subjected to Zeeman splitting, and interaction between light and magnetism caused by irradiating the glass cell with light obtained by changing the polarization state such as linearly polarized light, circularly polarized light or elliptically polarized light or light obtained by changing the intensity or the phase such as intensity modulated light or phase modulated light is used to detect the magnetism in the glass cell. The light source of pump light with which the glass cell is irradiated needs to be a light source with stabilized frequency to such an extent that is comparable to use in a field of spectroscopic measurement of atoms and molecules.
Various lasers including semiconductor lasers are used for light sources. A laser with frequency stabilization has a mechanism for detecting a frequency deviation from a reference deviation. A signal obtained by detecting a frequency deviation serves as a control signal for frequency stabilization of the laser. Absorption lines of atoms and molecules or an interferometer are used as a target to be the reference frequency.
Japanese Patent Application Laid-Open No. 2005-72103 (PTL 1) discloses a background art relating to frequency stabilization of a laser. PTL 1 discloses a technology for stabilizing a laser frequency utilizing the sub-Doppler spectrum of atoms, and that a simple and appropriate frequency stabilizing laser device as compared to conventional devices is provided.
The light intensities of light sources including semiconductor lasers are typically not always stable. Thus, in an application requiring a stable light intensity, automatic power control (APC) is conducted, in which light generated by a light source is measured by a photodetector, and the light intensity is stabilized by controlling drive current of the light source by the measurement signal.
Japanese Patent Application Laid-Open No. 2008-153320 (PTL 2) discloses a background art using the APC. PTL 2 discloses that a light source system including a light source, a photodetector configured to detect light output from the light source, a light scatterer disposed between optical paths of the light source and the photodetector, and a control/correction′ unit configured to control an output of the light source by using a detection result from the photodetector or correct an output variation of the light source is provided.
Japanese Patent Application Laid-Open No. 2002-314187 (PTL 3) also discloses a background art. PTL 3 discloses that a laser diode module is achieved, which includes an optical semiconductor element in which a semiconductor laser and an electro-absorption modulator are integrated, and in which an output of backlight from the semiconductor laser side is input to a photodiode via an optical filter having a wavelength transmission dependency, temperature control is conducted according to an output from the photodiode, and the current of the semiconductor laser is controlled on a basis of an output from the electro-absorption modulator.